I know this program was a little problematic, but it worked great for me. Is there a replacement in the tree? I've tried compiling it separately, but it doesn't find any of its prereqs, even though they are installed.

It appears to be available through several overlays: http://gpo.zugaina.org/media-gfx/gscan2pdf, although none of them appear to have the latest version shown on the homepage. You could create your own local ebuild using one of those available in the overlays as a template to get the 1.1.0 version, or go with 1.0.6 from the sabayon overlay.

As a alternative which is in the tree, I use media-gfx/simple-scan for that task.

In former times I also used gscan2pdf, but now simple-scan is well enough for me.
Does anybody who knows both, misses some function in simple-scan, just for interest.

After emerging the latest media-gfx/gscan2pdf-1.1.0 from the sabayon overlay, I went ahead and emerged the latest media-gfx/simple-scan-3.4.2, and I find that simple-scan also scans to PDF, is an easier tool to use, and gives more reliable scans. So I removed gscan2pdf in favor of simple-scan too.